Because there was never anything communist about these states in any way whatsoever.

Communism is a state (as in a social, political and economic condition, not a government). None of these states ever reached this condition, and, therefore, was never communist. And, one could argue, that their development literally went the opposite way to what could be called communist with a straight face. As the anarchist Bakunin famously said, "the people's boot is still a boot."

This is why the Maoist-types call this shit "democratic centralism," which is essentially just double-speak for "what the party says goes."

This does not make the idea of communism invalid - but it's still as perfectly vague as ever, unfortunately.